Regex Cheat Sheet — Complete Reference (PCRE, JavaScript, Python)

Everything you need to write and read regular expressions, on one page. Works for JavaScript, Python, PCRE, and most modern regex engines — dialect-specific notes are called out where they matter. Try any pattern immediately in the regex tester.

Character classes

Token	Matches
.	Any character except newline (add s flag for dotall)
\w	Word character: letters, digits, underscore
\W	Non-word character
\d	Digit (0-9)
\D	Non-digit
\s	Whitespace (space, tab, newline)
\S	Non-whitespace
[abc]	Any of a, b, or c
[^abc]	Anything except a, b, or c
[a-z]	Any lowercase letter
[A-Z]	Any uppercase letter
[0-9]	Any digit (same as \d)
\p{L}	Any Unicode letter (needs u flag)
\p{N}	Any Unicode number
\p{Emoji}	Any emoji (Unicode-aware engines)

Anchors & boundaries

Token	Matches
^	Start of string (or line with m flag)
$	End of string (or line with m flag)
\b	Word boundary
\B	Non-word boundary
\A	Start of string only (not per-line, even with m)
\Z	End of string only

Quantifiers

Token	Meaning
*	0 or more (greedy)
+	1 or more (greedy)
?	0 or 1 (optional)
{n}	Exactly n
{n,}	n or more
{n,m}	Between n and m
*? +? ?? {n,m}?	Lazy (non-greedy) versions — match as little as possible
*+ ++	Possessive (PCRE/Java only) — no backtracking

Groups & references

Syntax	Meaning
(abc)	Capturing group
(?:abc)	Non-capturing group (no backref slot)
(?<name>abc)	Named capturing group
\1 \2 …	Back-reference to group 1, 2, …
\k<name>	Back-reference to named group
$1 $2	Reference in replacement string
a|b	Alternation (a OR b)

Lookarounds

Syntax	Meaning	Example
(?=...)	Lookahead: followed by	\d+(?= dollars)
(?!...)	Negative lookahead	foo(?!bar)
(?<=...)	Lookbehind: preceded by	(?<=\$)\d+
(?<!...)	Negative lookbehind	(?<!no )good

Flags (modifiers)

Flag	Effect
g	Global — find all matches, not just first
i	Case-insensitive
m	Multi-line — ^/$ match line boundaries
s	Dotall — . matches newlines
u	Unicode — enables \p{...}
y	Sticky — match from lastIndex only (JS)
x	Extended — whitespace + comments inside pattern (PCRE, Python)

Ready-to-use patterns

Pattern	Regex
Email (practical)	[^\s@]+@[^\s@]+\.[^\s@]+
URL (http/https)	https?:\/\/\S+
IPv4	^((25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(25[0-5]|2[0-4]\d|[01]?\d\d?)$
UUID	^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$
Hex color	^#(?:[0-9a-fA-F]{3}){1,2}$
Date YYYY-MM-DD	^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$
Time HH:MM (24h)	^([01]\d|2[0-3]):[0-5]\d$
US phone	^\+?1?[\s.-]?\(?\d{3}\)?[\s.-]?\d{3}[\s.-]?\d{4}$
Password (8+ with mix)	^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$
Positive integer	^[1-9]\d*$
Integer (with sign)	^-?\d+$
Float	^-?\d+(\.\d+)?$
Slug (kebab-case)	^[a-z0-9]+(?:-[a-z0-9]+)*$
Leading/trailing whitespace	^\s+|\s+$
Consecutive duplicates	(.)\1+
HTML tag (simple)	<\/?[a-zA-Z][^>]*>
Credit card (generic)	^\d{4}[ -]?\d{4}[ -]?\d{4}[ -]?\d{4}$
Extract quoted string	"([^"\\]*(?:\\.[^"\\]*)*)"

Common gotchas

• Greedy by default. .* matches as much as possible; use .*? for the shortest match.
• Escape specials: . * + ? ( ) [ ] { } | \ ^ $ / need a backslash when literal.
• Inside character classes most specials lose their meaning; only - ] \ and ^ (at start) need escaping.
• Don't parse HTML with regex — use a parser. Regex handles flat text, not nested markup.
• Lookbehind limits — JavaScript supports variable-width lookbehind since 2018; older engines require fixed width.
• Performance trap: nested quantifiers like (a+)+ cause catastrophic backtracking. Use possessive quantifiers or rewrite the pattern.

Test any pattern

Drop any pattern from this page into the regex tester — matches highlight live, capture groups extract separately, and flags toggle from a menu. For diffing two files line-by-line (a common regex alternative), use diff checker. For side-by-side text comparison, text compare.

Bookmark this page for daily use, or print it as a single-sheet reference. Patterns here work in JavaScript (RegExp), Python (re), Ruby, PHP (preg_*), and most PCRE engines with the caveats noted.